Abstract:

An optical modulation device includes: an optical splitter for splitting
input light into a first input beam and a second input beam; an optical
intensity modulator for modulating the intensity of the first input beam
in response to a modulating signal; a variable optical phase shifter for
shifting the phase of the second input beam; and an optical combiner for
combining an output beam of the optical intensity modulator and an output
beam of the variable optical phase shifter into a combined beam and
outputting the combined beam. The amount of phase shift produced by the
variable optical phase shifter is externally controlled.

Claims:

1. An optical modulation device comprising:an optical splitter for
splitting input light into a first input beam and a second input beam;an
optical intensity modulator for modulating intensity of the first input
beam in response to a modulating signal;a variable optical phase shifter
for shifting phase of the second input beam; andan optical combiner for
combining an output beam of said optical intensity modulator and an
output beam of said variable optical phase shifter into a combined beam
and outputting the combined beam, wherein phase shift produced by said
variable optical phase shifter is controlled externally to said optical
modulation device.

2. The optical modulation device as claimed in claim 1, wherein the output
beam of said optical intensity modulator has higher intensity than the
output beam of said variable optical phase shifter when combined by said
optical combiner.

3. The optical modulation device as claimed in claim 1, wherein:said
variable optical phase shifter includes a semiconductor laser that
includes a waveguide;the second input beam travels through said
waveguide; andthe refractive index of said waveguide is varied by varying
the current injected into said variable optical phase shifter.

5. The optical modulation device as claimed in claim 1, further comprising
a variable optical attenuator for attenuating the intensity of the second
input beam, wherein attenuation produced by said variable optical
attenuator is controlled externally of said optical modulation device.

6. The optical modulation device as claimed in claim 1, wherein either
ratio between the first and second input beams emerging from said optical
splitter, or ratio at which said optical combiner combines the output
beam of said optical intensity modulator and the output beam of said
variable optical phase shifter is controlled externally of said optical
modulation device.

7. The optical modulation device as claimed in claim 1, wherein said
variable optical phase shifter operates in synchronism with the
modulating signal applied to said optical intensity modulator.

8. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 1; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less temperature dependent than the output beam of said optical intensity
modulator by varying, in accordance with ambient temperature, the phase
shift produced by said variable optical phase shifter.

9. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 5; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less temperature dependent than the output beam of said optical intensity
modulator by varying, in accordance with ambient temperature, at least
one of the phase shift produced by said variable optical phase shifter,
and attenuation produced by said variable optical attenuator, or both.

10. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 6; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less temperature dependent than the output beam of said optical intensity
modulator by varying at least one of the following in accordance with
ambient temperature:the phase shift produced by said variable optical
phase shifter,the ratio between the first and second input beams emerging
from said optical splitter, andthe ratio at which said optical combiner
combines the output beam of said optical intensity modulator and the
output beam of said variable optical phase shifter.

11. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 1; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less wavelength dependent than the output beam of said optical intensity
modulator by varying, in accordance with the wavelength of the input
light, the phase shift produced by said variable optical phase shifter.

12. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 5; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less wavelength dependent than the output beam of said optical intensity
modulator by varying, in accordance with the wavelength of the input
light, at least one of the phase shift produced by said variable optical
phase shifter, and of the attenuation produced by said variable optical
attenuator.

13. An optical semiconductor device comprising:the optical modulation
device as claimed in claim 6; anda control circuit for controlling said
optical modulation device, wherein said control circuit adjusts a
characteristic of the output beam of said optical modulation device to be
less wavelength dependent than the output beam of said optical intensity
modulator by varying at least one of the following in accordance with the
wavelength of the input light:the phase shift produced by said variable
optical phase shifter,the ratio between the first and second input beams
emerging from said optical splitter, andthe ratio at which said optical
combiner combines the output beam of said optical intensity modulator and
the output beam of said variable optical phase shifter.

14. The optical semiconductor device as claimed in claim 11, further
comprising a variable wavelength laser monolithically or hybrid
integrated with said optical modulation device.

15. The optical modulation device as claimed in claim 1, wherein:said
variable optical phase shifter includes a semiconductor optical amplifier
that includes a waveguide;the second input beam travels through said
waveguide; andthe refractive index of said waveguide is varied by varying
the current injected into said variable optical phase shifter.

16. The optical modulation device as claimed in claim 1, wherein said
optical intensity modulator is a Mach-Zehnder optical modulator.

17. The optical modulation device as claimed in claim 2, further
comprising a variable optical attenuator for attenuating the intensity of
the second input beam, wherein the attenuation produced by said variable
optical attenuator is controlled externally of said optical modulation
device.

18. The optical modulation device as claimed in claim 3, further
comprising a variable optical attenuator for attenuating the intensity of
the second input beam, wherein the attenuation produced by said variable
optical attenuator is controlled externally of said optical modulation
device.

19. The optical modulation device as claimed in claim 4, further
comprising a variable optical attenuator for attenuating the intensity of
the second input beam, wherein the attenuation produced by said variable
optical attenuator is controlled externally of said optical modulation
device.

20. The optical modulation device as claimed in claim 15, further
comprising a variable optical attenuator for attenuating the intensity of
the second input beam, wherein the attenuation produced by said variable
optical attenuator is controlled externally of said optical modulation
device.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an optical modulation device which
receives a light beam and modulates its intensity, and more particularly
to an optical modulation device adapted to allow control of the chirp
characteristics of the output beam.

[0003]2. Background Art

[0004]The three types of optical modulators capable of high-speed
modulation (10 Gbit/s or more) which have been used in practical
applications are: (1) the lithium niobate (LiNbO3) Mach-Zehnder
optical modulator, (2) the semiconductor Mach-Zehnder optical modulator,
and (3) the electro absorption optical modulator. Lithium niobate
Mach-Zehnder optical modulators are most widely used as modulation light
sources in optical transponders, since their performance varies only a
little with temperature and wavelength variations and they have stable
chirp characteristics.

[0005]In a lithium niobate Mach-Zehnder optical modulator, the incident
light beam is divided equally by an optical splitter into two beams which
are then passed through their respective waveguides. At that time,
modulating signals of equal amplitude but 180° out of phase are
respectively applied to these waveguides to change their refractive
indices and thereby change the phases of the waveguide beams by
±90°, respectively. The waveguide beams are then combined by an
optical combiner and output from the modulator, thus converting the phase
modulation into intensity modulation.

[0006]Lithium niobate Mach-Zehnder optical modulators typically have a
waveguide length of 30-50 mm; that is, optical semiconductor devices
incorporating this type of optical modulator must be as much as 50-100 mm
in length. Although prior art optical transponders (dimensioned 5 inches
by 7 inches, or 4.5 inches by 3.5 inches) have a space for accommodating
such an optical semiconductor device(s), there is no such space available
in XFP (10 Gigabit Form Factor Pluggable) optical transceivers, which
have been recently used in response to the decreasing size of optical
communications devices. It is not possible to sufficiently reduce the
size of lithium niobate Mach-Zehnder optical modulators, since
application of a voltage to LiNbO3 results in only a small change in
its refractive index (the actual amount of change being determined by the
material constants). As a result, this type of optical modulator must
have a length on the order of a few tens of millimeters or more (as
described above) to introduce a 90° phase change in the beams
traveling through its waveguides.

[0007]In the case of a semiconductor Mach-Zehnder optical modulator, on
the other hand, the modulator can cause ±90° phase changes in
the beams propagating through its semiconductor optical waveguides even
if the waveguides are as short as approximately a few millimeters in
length, provided that they have a band gap wavelength approximately 100
nm shorter than the wavelength of the incident light. Such semiconductor
Mach-Zehnder optical modulators have proven to function satisfactorily.
Therefore, the size of semiconductor Mach-Zehnder optical modulators can
be reduced, making them suitable for use in XFP optical transceivers.
Furthermore, a semiconductor Mach-Zehnder optical modulator may be formed
from a material used to form an optical communications laser (e.g.,
InGaAsP on an InP substrate). This enables the optical modulator to be
integrally and monolithically formed with the optical communications
laser, resulting in a simplified optical system and hence reduced cost.
It should be noted that the performance of semiconductor Mach-Zehnder
optical modulators is more susceptible to temperature and wavelength
variations than the performance of lithium niobate Mach-Zehnder optical
modulators but less susceptible than the performance of electro
absorption optical modulators. Therefore, semiconductor Mach-Zehnder
optical modulators are a promising optical modulator that can be combined
with a variable wavelength laser to provide a next-generation small size
variable wavelength modulation light source.

[0008]However, the length of semiconductor Mach-Zehnder optical modulators
(approximately a few millimeters) is still too large to form them in a
sufficient quantity on a compound semiconductor wafer, resulting in
increased manufacturing cost. (For example, InP wafers are 2-3 inches in
diameter.) On the other hand, electro absorption optical modulators can
be approximately 0.2 mm long, with an extinction ratio of approximately
10 dB, for example. Furthermore, they can be easily monolithically
integrated with a semiconductor laser and are often used in fixed
wavelength XFP transceivers.

SUMMARY OF THE INVENTION

[0009]The optical loss in a semiconductor optical waveguide includes
wavelength independent components, which are caused by the irregularities
on the sides of the waveguide, etc., and wavelength dependent components,
which are caused by interband absorption, free carrier absorption, etc.
The band gap wavelength of the semiconductor optical waveguide is set
relatively close to the wavelength of the incident light (namely,
approximately 100 nm away from the incident light wavelength) in order to
cause a large change in its refractive index when a voltage is applied to
the waveguide. Therefore, the optical loss and the effective refractive
index of the semiconductor optical waveguide tend to vary with variations
in the amount of interband absorption, etc. resulting from variations in
the in-plane composition of the semiconductor wafer, for example. That
is, in the case of an optical modulator employing semiconductor optical
waveguides (namely, one upper and one lower semiconductor optical
waveguide), it is difficult to accurately control the optical loss and
the effective refractive index of these semiconductor optical waveguides,
resulting in significant variations in the chirp characteristics of the
output light beam.

[0010]Further, it is required that semiconductor Mach-Zehnder optical
modulators and electro absorption optical modulators be operated in a
substantially constant temperature environment. That is, since the band
gap of semiconductor material varies with temperature, the
characteristics of these optical modulators also vary with temperature.
Therefore, these modules are often used in combination with a peltiert
device to maintain their temperature constant. However, peltiert devices
have high power consumption.

[0011]A modulator integrated laser has been proposed in which a
semiconductor laser is monolithically integrated with an electro
absorption optical modulator made of AlGaInAs and the bias voltage is
adjusted based on temperature. This modulator integrated laser is
suitable for use in optical transceivers which, in order to reduce power
consumption, do not employ a peltiert device. It has been verified that
the use of this laser allows for 10 Gbit/s transmission over a wide
temperature range [see, e.g., Makino et al., Proceedings of the Optical
Fiber Communication Conference (OFC2007), No. OMS1]. In order to use such
a modulator integrated laser in practical applications, however, it is
necessary to solve problems such as a significant reduction in the
optical output at high temperatures and difficulty in ensuring long term
reliability. (It should be noted that it is usually difficult to ensure
that optical devices containing Al exhibit long term reliability.)

[0012]Thus, optical transceivers which do not employ a peltiert device
(which has considerable power consumption) are limited to those in which
the laser is directly modulated. However, direct modulation allows the
production of good waveforms only at bit rates of 10 Gbit/s or less. For
this reason there has yet to be developed a low power consumption optical
transceiver having a bit rate of 40 Gbit/s.

[0013]Further, since the band gap wavelength of electro absorption optical
modulators is set relatively close to the wavelength of the incident
light (namely, approximately 50 nm away from the incident light
wavelength), the absorption coefficient and the refractive index vary
significantly with wavelength variations, which has prevented the
modulators from being used in combination with a variable wavelength
laser.

[0014]The present invention has been devised to solve the above problems.
It is, therefore, a first object of the present invention to provide an
optical modulation device adapted to allow control of the chirp
characteristics of its output beam.

[0015]A second object of the present invention is to provide a small size,
low power consumption, yet low-cost optical semiconductor device capable
of operation at a high modulation bit rate and exhibiting the desired
performance independently of the ambient temperature.

[0016]A third object of the present invention is to provide an optical
semiconductor device capable of exhibiting the desired performance
independently of wavelength.

[0017]According to a first aspect of the present invention, an optical
modulation device comprises: an optical splitter for splitting input
light into a first input beam and a second input beam; an optical
intensity modulator for modulating the intensity of said first input beam
in response to a modulating signal; a variable optical phase shifter for
shifting the phase of said second input beam; and an optical combiner for
combining an output beam of said optical intensity modulator and an
output beam of said variable optical phase shifter into a combined beam
and outputting said combined beam; wherein said optical modulation device
is adapted to allow external control of the amount of phase shift
produced by said variable optical phase shifter.

[0018]According to a second aspect of the present invention, an optical
semiconductor device comprising: the optical modulation device according
to the first aspect of the present invention; and a control circuit for
controlling said optical modulation device; wherein said control circuit
adjusts a characteristic of said output beam of said optical modulation
device to be less temperature dependent than that of said output beam of
said optical intensity modulator by varying, in accordance with ambient
temperature, said amount of phase shift produced by said variable optical
phase shifter.

[0019]According to a third aspect of the present invention, an optical
semiconductor device comprising: the optical modulation device according
to the first aspect of the present invention; and a control circuit for
controlling said optical modulation device; wherein said control circuit
adjusts a characteristic of said output beam of said optical modulation
device to be less wavelength dependent than that of said output beam of
said optical intensity modulator by varying, in accordance with the
wavelength of said input light, said amount of phase shift produced by
said variable optical phase shifter.

[0020]Thus, the first aspect of the present invention can provide an
optical modulation device adapted to allow control of the chirp
characteristics of its output beam.

[0021]Further, the second aspect can provide a small size, low power
consumption, yet low-cost optical semiconductor device capable of
operation at a high modulation bit rate and exhibiting the desired
performance independently of the ambient temperature.

[0022]Further, the third aspect of the present invention can provide an
optical semiconductor device capable of exhibiting the desired
performance independently of wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagram showing an optical modulation device according
to a first embodiment of the present invention.

[0024]FIG. 2 is a diagram showing the chirp characteristics of the optical
intensity modulator 16 shown in FIG. 1.

[0025]FIG. 3 is a diagram showing the electric field vector of the output
beam of the optical intensity modulator.

[0026]FIG. 4 is a diagram showing the electric field vector of the output
beam from the optical modulation device of the present embodiment.

[0027]FIG. 5 is an enlarged view of the origin area of FIG. 4.

[0028]FIG. 6 shows for reference the electric field vector of the output
beam of the optical modulation device when the variable optical phase
shifter is adapted to output a beam of relatively high intensity.

[0030]FIG. 8 shows the chirp characteristics of the output beams from the
optical intensity modulator and from the optical modulation device when
the device is set to the above first parameter setting.

[0032]FIG. 10 shows the chirp characteristics of the output beams from the
optical intensity modulator and from the optical modulation device when
the device is set to the above second parameter setting.

[0033]FIG. 11 is a diagram showing an optical modulation device according
to a second embodiment of the present invention.

[0034]FIG. 12 is a diagram showing an optical modulation device according
to a third embodiment of the present invention.

[0035]FIG. 13 is a diagram showing an optical modulation device 10
according to a fourth embodiment of the present invention.

[0036]FIG. 14 is a diagram showing an optical semiconductor device
according to a fifth embodiment of the present invention.

[0037]FIG. 15 is a graph showing the temperature dependence of the chirp
characteristics of the optical intensity modulator.

[0038]FIG. 16 shows plots of the electric field vector of the output beam
from the optical intensity modulator at different temperatures.

[0039]FIG. 17, on the other hand, shows plots of the electric field vector
of the output beam from the optical modulation device of the present
embodiment at different temperatures.

[0040]FIG. 18 is a graph showing the temperature dependence of the chirp
characteristics of the optical modulation device of the present
embodiment.

[0041]FIG. 19 is a diagram showing an optical semiconductor device
according to a sixth embodiment of the present invention.

[0042]FIG. 20 is a diagram showing an optical semiconductor device
according to a seventh embodiment of the present invention.

[0043]FIG. 21 is a diagram showing an optical semiconductor device
according to an eighth embodiment of the present invention.

[0044]FIG. 22 is a diagram showing an optical semiconductor device
according to a ninth embodiment of the present invention.

[0045]FIG. 23 is a diagram showing an optical semiconductor device
according to a tenth embodiment of the present invention.

[0046]FIG. 24 is a diagram showing an optical semiconductor device
according to an eleventh embodiment of the present invention.

[0047]FIG. 25 is a diagram showing an optical semiconductor device
according to a twelfth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

[0048]FIG. 1 is a diagram showing an optical modulation device according
to a first embodiment of the present invention. The optical modulation
device 10, includes an input optical waveguide 12, an optical splitter
14, an optical intensity modulator 16, a variable optical phase shifter
18, an optical combiner 20, and an output optical waveguide 22. The
optical splitter 14 splits the input light beam received through the
input optical waveguide 12 into first and second input beams.

[0049]The optical intensity modulator 16 modulates the intensity of the
first input beam in response to a modulating signal received from an
external modulator driver 24. The optical intensity modulator 16 maybe,
for example, a 200 μm long electro absorption optical modulator.
Electro absorption optical modulators can be typically as short as 0.3 mm
or less in length, meaning that the size of the optical intensity
modulator 16 can be reduced.

[0050]The variable optical phase shifter 18 shifts the phase of the second
input beam in response to a current received from an external current
source 26. That is, the amount of phase shift produced by the variable
optical phase shifter 18 can be externally controlled. The variable
optical phase shifter 18 may be, for example, a semiconductor laser or
semiconductor optical amplifier adapted such that a variation in the
current injected into its active layer (a waveguide) results in a
variation in the refractive index of the layer. It should be noted that
semiconductor lasers and semiconductor optical amplifiers can be formed
relatively easily. Further, varying the amount of current injected into
the active layer (or waveguide) can cause a large change in its
refractive index, even with a short device length, as compared to
applying a reverse bias voltage to the active layer. According to the
present embodiment, a DC current is applied to the variable optical phase
shifter 18, thereby precluding the problem of high frequency response.
Therefore, a semiconductor optical amplifier capable of providing a large
refractive index change and hence a large phase shift is used as the
variable optical phase shifter 18. It should be noted that the variable
optical phase shifter 18 may be a device with a waveguide adapted such
that a voltage can be externally applied to the waveguide so as to vary
substantially only the real part of its refractive index.

[0051]The optical combiner 20 combines the output beams from the optical
intensity modulator 16 and from the variable optical phase shifter 18 and
outputs the combined beam through the output optical waveguide 22. The
(intensity) ratio of the first input beam to the second input beam
emerging from the optical splitter 14 is such that the optical path from
the input optical waveguide 12 to the optical intensity modulator 16 has
a smaller insertion loss than the optical path from the input optical
waveguide 12 to the variable optical phase shifter 18. Further, the
optical combiner 20 combines the output beams from the optical intensity
modulator 16 and from the variable optical phase shifter 18 in such a
ratio that the optical path from the optical intensity modulator 16 to
the output optical waveguide 22 has a smaller insertion loss than the
optical path from the variable optical phase shifter 18 to the output
optical waveguide 22. This ensures that the output beam from the optical
intensity modulator 16 has higher intensity than the output beam from the
variable optical phase shifter 18 when these beams are combined by the
optical combiner 20.

[0052]FIG. 2 is a diagram showing the chirp characteristics of the optical
intensity modulator 16 shown in FIG. 1. The chirp characteristics
correspond to the dependence of the a parameter (described below) of the
optical intensity modulator 16 on the modulating signal, i.e., the
reverse bias voltage applied to the optical intensity modulator 16. An a
parameter is the ratio of the variation of the real part (Δn') to
the variation of the imaginary part (Δn'') of the complex
refractive index of a waveguide (or optical modulator) when a small
modulating signal is applied to the waveguide, as expressed by the
following equation:

[0053]FIG. 3 is a diagram showing the electric field vector of the output
beam of the optical intensity modulator. The horizontal axis represents
the real part of the electric field vector, and the vertical axis
represents the imaginary part. That is, FIG. 3 shows variations in the
intensity and the phase of the output beam of the optical intensity
modulator 16. This graph is normalized such that point A at (1,0)
coordinates represents the optical intensity and phase of the output beam
of the optical intensity modulator 16 when the modulating signal is at 0
V. When the optical intensity modulator 16 transitions from its ON state
to its OFF state, the tip of the electric field vector, E, of the output
beam from the optical intensity modulator 16 traces out a curve AB, that
is, moves from point A to point B along the curve shown in FIG. 3.

[0054]The electric field vector E is expressed by the following equation:

E=(Ecos F, Esin F) (Eq. 2)

where F is the angle of the electric field vector E relative to the
horizontal or real axis (i.e., the phase of the output beam). The square
of the length of the electric field vector E (i.e., |E|2)
corresponds to the intensity of the output beam, P.

[0055]The following equation relates the a parameter to the phase F:

φ t = α 2 1 P P t ( Eq .
3 ) ##EQU00002##

The curve AB shown in FIG. 3 was obtained from the chirp characteristics
of FIG. 2 by using Eq. 3.

[0056]FIG. 4 is a diagram showing the electric field vector of the output
beam from the optical modulation device of the present embodiment. FIG. 5
is an enlarged view of the origin area of FIG. 4.

[0057]The electric field vector E3 of the output beam from the output
optical waveguide 22 is the sum of the electric field vector E1 of
the output beam from the optical intensity modulator 16 and the electric
field vector E2 of the output beam from the variable optical phase
shifter 18. When the optical modulation device 10 transitions from its ON
state to its OFF state, the tip of the electric field vector E1 of
the output beam from the optical intensity modulator 16 traces out a
curve AB indicated by a broken line in FIG. 4. At that time, the tip of
the electric vector E3 of the output beam from the output optical
waveguide 22 traces out a curve A'B' indicated by the solid line in FIG.
5. That is, the chirp characteristics of the output beam of the optical
modulation device 10 differ from those of the output beam of the optical
intensity modulator 16.

[0058]The current injected into the variable optical phase shifter 18 may
be varied to vary the refractive index of the shifter and thereby adjust
the angle (or phase) F of the electric field vector E2. That is, it
is possible to externally adjust the amount of phase shift produced by
the variable optical phase shifter 18 and thereby to control the chirp
characteristics of the output beam of the optical modulation device 10.
This allows the optical modulation device 10 to output a beam having the
desired chirp characteristics even when the output beam of the optical
intensity modulator 16 does not have intended chirp characteristics. In
this way, the chirp characteristics of the output beam of the optical
modulation device 10 may be varied in response to variations in the
operating conditions such as temperature, the intensity and wavelength of
the input light beam, the dispersion strength of the fiber, and the
polarization.

[0059]Further, since the optical path including the optical intensity
modulator 16 has a smaller insertion loss than the optical path including
the variable optical phase shifter 18, the beam input to the variable
optical phase shifter 18 has lower intensity than the beam input to the
optical intensity modulator 16. This arrangement results in a reduction
in the total insertion loss, as compared to when the optical modulation
device 10 does not include the variable optical phase shifter 18.

[0060]Further, the output beam of the optical intensity modulator 16 has
higher intensity than the output beam of the variable optical phase
shifter 18 when these beams are combined by the optical combiner 20. In
other words, the electric field vector E2 (of the output beam from
the variable optical phase shifter 18) is shorter than the electric field
vector E1 (of the output beam from the optical intensity modulator
16). Therefore, for example, when the optical modulation device 10 is in
its ON state, the optical intensity of the output beam of the output
optical waveguide 22 (represented by point A' in FIG. 4) differs only
slightly from the optical intensity of the output beam of the optical
intensity modulator 16 (represented by point A in FIG. 4). That is,
regardless of the amount of phase shift produced by the variable optical
phase shifter 18, the optical intensity of the output beam of the output
optical waveguide 22 is always higher when the optical modulation device
10 is in its ON state than when it is in its OFF state (represented by
point B' in FIG. 5), resulting in a high extinction ratio. This
eliminates the occurrence of an erroneous bit and thereby makes the
optical modulation device of the present embodiment suitable for use in
optical data communications.

[0061]FIG. 6 shows for reference the electric field vector of the output
beam of the optical modulation device when the variable optical phase
shifter is adapted to output a beam of relatively high intensity.
Referring to FIG. 6, when the optical modulation device transitions from
its ON state to its OFF state, the tip of the electric field vector
E1 of the output beam from the optical intensity modulator 16 traces
out a curve AB (indicated by the broken line) and the tip of the electric
field vector E3 of the output beam from the output optical waveguide
22 traces out a curve A''B'' (indicated by a solid line). As shown, for
example, when the optical modulation device is in its ON state, the
optical intensity of the output beam of the output optical waveguide 22
(represented by point A'' in FIG. 6) differs significantly from the
optical intensity of the output beam of the optical intensity modulator
16 (represented by point A in FIG. 6), since the optical intensity of the
output beam from the variable optical phase shifter 18 is increased
(i.e., the length of the electric field vector E2 is increased).
That is, depending on the amount of phase shift produced by the variable
optical phase shifter 18, the optical intensity of the output beam of the
output optical waveguide 22 may be higher when the optical modulation
device is in its OFF state than when it is in its ON state, resulting in
the occurrence of an erroneous bit.

[0062]FIG. 7 shows the electric field vector E1 of the output beam of
the optical intensity modulator 16 and the electric field vector E3
of the output beam of the optical modulation device 10 when the electric
field vector E2 of the output beam of the variable optical phase
shifter 18 has a magnitude of 0.1 and an angle of 2.4 radians as measured
in a counterclockwise direction from the positive real axis (first
parameter setting). Referring to FIG. 7, when the tip of the electric
field vector E1 traces out a curve AB (indicated by the broken
line), the tip of the electric field vector E3 traces out a curve
A'B' (indicated by the solid line). FIG. 8 shows the chirp
characteristics of the output beams from the optical intensity modulator
and from the optical modulation device when the device is set to the
above first parameter setting. As shown, the output beam of the optical
modulation device 10 has a smaller a parameter than the output beam of
the optical intensity modulator 16 regardless of the modulating signal
voltage level. This means that in this case the optical modulation device
can be used to improve the transmission characteristics of positive
dispersion fiber, as compared to an optical modulation device which does
not include a variable optical phase shifter.

[0063]FIG. 9 shows the electric field vector E1 of the output beam of
the optical intensity modulator 16 and the electric field vector E3
of the output beam of the optical modulation device 10 when the electric
field vector E2 of the output beam of the variable optical phase
shifter 18 has a magnitude of 0.1 and an angle of 5.2 radians as measured
in a counterclockwise direction from the positive real axis (second
parameter setting). Referring to FIG. 9, when the tip of the electric
field vector E1 traces out a curve AB (indicated by the broken
line), the tip of the electric field vector E3 traces out a curve
A'B' (indicated by the solid line). FIG. 10 shows the chirp
characteristics of the output beams from the optical intensity modulator
and from the optical modulation device when the device is set to the
above second parameter setting. As shown, the output beam of the optical
modulation device has a larger a parameter than the output beam of the
optical intensity modulator 16 regardless of the modulating signal
voltage level. This means that in this case the optical modulation device
can be used to improve the transmission characteristics of negative
dispersion fiber, as compared to an optical modulation device which does
not include a variable optical phase shifter.

[0064]Although the optical intensity modulator 16 of the present
embodiment has been described as an electro absorption optical modulator,
it is to be understood that other types of optical modulators such as a
Mach-Zehnder optical modulator may be used instead of an electro
absorption optical modulator. That is, the use of a Mach-Zehnder optical
modulator (adapted according to the present embodiment) allows the
extinction ratio and chirp characteristics of the optical modulation
device to be corrected or improved in the same manner as described above,
even in modulation systems, such as CS-RZ, DPSK, and DQPSK, which cannot
be directly accommodated by an electro absorption optical modulator.

[0065]Further, although the optical splitter 14 and the optical combiner
20 of the present embodiment have been described as a Y-shaped splitting
waveguide and a Y-shaped combining waveguide, respectively, it is to be
understood that multimode interference (MMI) waveguides having the same
functions as the splitting and combining waveguides may be used instead.

Second Embodiment

[0066]FIG. 11 is a diagram showing an optical modulation device according
to a second embodiment of the present invention. It should be noted that
those components common to the first embodiment retain the same reference
numerals and will not be further described.

[0067]The optical modulation device, 10, of the present embodiment differs
from that of the first embodiment in that it additionally includes a
variable optical attenuator 28 for attenuating the intensity of the
second input beam. The insertion loss of the variable optical attenuator
28 varies in accordance with the voltage applied thereto by an external
power supply 30. That is, the amount of attenuation produced by the
variable optical attenuator 28 can be externally controlled.

[0068]Therefore, it is possible to vary the intensity of the output beam
of the variable optical phase shifter 18, as well as the angle (or phase)
of the electric field vector E2 of the beam, thereby enabling the
chirp characteristics of the output beam of the optical modulation device
10 to be adjusted over a wider range than in the first embodiment.

Third Embodiment

[0069]FIG. 12 is a diagram showing an optical modulation device according
to a third embodiment of the present invention. It should be noted that
those components common to the first embodiment retain the same reference
numerals and will not be further described.

[0070]The optical modulation device, 10, of the present embodiment differs
from that of the first embodiment in that the optical splitter 14 and the
optical combiner 20 are replaced by a variable branching ratio input
multimode interference (MMI) coupler 32 and a variable branching ratio
output multimode interference coupler 34, respectively. These couplers 32
and 34 function as an optical splitter and an optical combiner,
respectively. The variable branching ratio input multimode interference
coupler 32 and the variable branching ratio output multimode interference
coupler 34 may be of the type described in Leuthold et al., JOURNAL OF
LIGHTWAVE TECHNOLOGY, VOL. 19, NO. 5, pp. 700-707, MAY 2001.

[0071]The variable branching ratio input multimode interference coupler 32
varies the ratio between the first input beam and the second input beam
in response to a voltage received from an external power supply 36. The
variable branching ratio output multimode interference coupler 34, on the
other hand, varies the ratio in which the output beams from the optical
intensity modulator 16 and from the variable optical phase shifter 18 are
combined, in response to a voltage received from an external power supply
38. Thus, the variable branching ratio input multimode interference
coupler 32 allows external adjustment of the ratio between the first and
second input beams, while the variable branching ratio output multimode
interference coupler 34 allows external adjustment of the ratio in which
to combine the output beams from the optical intensity modulator 16 and
from the variable optical phase shifter 18.

[0072]Therefore, it is possible to vary the intensity of the output beam
of the variable optical phase shifter 18, as well as the angle (or phase)
of the electric field vector E2 of the beam, thereby enabling the
chirp characteristics of the output beam of the optical modulation device
10 to be adjusted over a wider range than in the first embodiment.
Further, the present embodiment can reduce the insertion loss of the
optical modulation device 10, as compared to the second embodiment.

Fourth Embodiment

[0073]FIG. 13 is a diagram showing an optical modulation device 10
according to a fourth embodiment of the present invention. It should be
noted that those components common to the first and second embodiments
retain the same reference numerals and will not be further described.

[0074]The optical modulation device 10 of the present embodiment differs
from that of the second embodiment in that the variable optical phase
shifter 18 is driven by the external modulator driver 24 or another
modulating signal source 40 such that the shifter operates at high speed
in sync with the modulating signal supplied to the optical intensity
modulator 16. This allows the optical modulation device 10 to exhibit the
desired chirp characteristics both in its ON and OFF states. Further, the
transient response characteristics of the optical modulation device can
be adjusted such that the device produces an optimum transient response
to the modulating signal which is applied to the device to correct its
chirp characteristics.

[0075]The modulating signal for driving the variable optical phase shifter
18 may have the same or opposite polarity as the modulating signal from
the modulator driver 24. Further, the modulating signal source 40 may
include a filter circuit for delaying the transient response or for
causing overshoot in the response.

Fifth Embodiment

[0076]FIG. 14 is a diagram showing an optical semiconductor device
according to a fifth embodiment of the present invention. The optical
semiconductor device, 42, includes an optical modulation device 10, a
semiconductor laser 44, a modulator driver 24, a thermistor 46, and a
control circuit 48.

[0077]The optical modulation device 10 has the same configuration as in
the third embodiment. The optical modulation device 10 and the
semiconductor laser 44 are monolithically or hybrid integrated together.
The modulator driver 24 supplies a modulating signal to the optical
intensity modulator 16 of the optical modulation device 10 in response to
an externally generated input electrical signal.

[0078]The thermistor 46 detects the ambient temperature and sends a signal
indicative thereof to the control circuit 48. The control circuit 48
adjusts the output beam of the optical modulation device 10 such that its
chirp characteristics are less temperature dependent than the chirp
characteristics of the output beam of the optical intensity modulator 16.
This is accomplished by varying at least one of the following in
accordance with the ambient temperature: the amount of phase shift
produced by the variable optical phase shifter 18; the ratio of the first
input beam to the second input beam emerging from the optical splitter
14; and the ratio at which the optical combiner 20 combines the output
beams from the optical intensity modulator 16 and from the variable
optical phase shifter 18.

[0079]FIG. 15 is a graph showing the temperature dependence of the chirp
characteristics of the optical intensity modulator. As shown, the chirp
increases with decreasing temperature. FIG. 16 shows plots of the
electric field vector of the output beam from the optical intensity
modulator at different temperatures. FIG. 17, on the other hand, shows
plots of the electric field vector of the output beam from the optical
modulation device of the present embodiment at different temperatures.
These figures indicate that the phase and/or intensity of the output beam
of the optical intensity modulator 16 may be adjusted by the control
circuit 48 based on the ambient temperature to reduce the temperature
dependence of the chirp characteristics of the output beam of the optical
modulation device 10.

[0080]FIG. 18 is a graph showing the temperature dependence of the chirp
characteristics of the optical modulation device of the present
embodiment. This graph is prepared using the data shown in FIG. 16. As
shown, the chirp characteristics of the optical modulation device 10 are
less temperature dependent than those of the optical intensity modulator
16.

[0081]Therefore, the optical semiconductor device 42 of the present
embodiment achieves the desired performance characteristics independently
of the ambient temperature. Furthermore, the device does not require a
peltiert device to maintain its temperature constant, resulting in lower
cost, lower power consumption, and smaller size. Further, the optical
semiconductor device 42 can achieve a modulation bit rate of 40 Gbit/s or
more, which is difficult to achieve by direct modulation. This makes the
optical semiconductor device 42 suitable for use in ultrahigh speed
optical fiber data communications.

[0082]Further, the optical modulation device 10 may be formed of a
semiconductor material having a high degree of long term reliability,
such as InGaAsP, with the result that not only are the chirp
characteristics of the device little temperature dependent, but also it
is capable of reliable operation over an extended period of time.

[0083]It should be noted that the optical modulation device 10 of the
present embodiment may have the same configuration as in the first
embodiment. In such a case, the control circuit 48 adjusts the amount of
phase shift produced by the variable optical phase shifter 18 based on
the ambient temperature such that the chirp characteristics of the output
beam of the optical modulation device 10 are less temperature dependent
than those of the output beam of the optical intensity modulator 16.

[0084]Further, the optical modulation device 10 of the present embodiment
may have the same configuration as in the second embodiment. In such a
case, the control circuit 48 adjusts the amount of phase shift produced
by the variable optical phase shifter 18 and/or the amount of attenuation
produced by the variable optical attenuator 28 based on the ambient
temperature such that the chirp characteristics of the output beam of the
optical modulation device 10 are less temperature dependent than those of
the output beam of the optical intensity modulator 16.

Sixth Embodiment

[0085]FIG. 19 is a diagram showing an optical semiconductor device
according to a sixth embodiment of the present invention. It should be
noted that those components common to the fifth embodiment retain the
same reference numerals and will not be further described.

[0086]The optical semiconductor device of the present embodiment differs
from that of the fifth embodiment in that the semiconductor laser 44 is
replaced by a variable wavelength laser 50 with the capability of varying
its wavelength, wherein the variable wavelength laser 50 is
monolithically or hybrid integrated with the optical modulation device
10. The control circuit 48 adjusts the oscillation wavelength of the
variable wavelength laser 50 to a wavelength specified externally. The
control circuit 48 also adjusts the output beam of the optical modulation
device 10 such that its chirp characteristics are less wavelength
dependent than the chirp characteristics of the output beam of the
optical intensity modulator 16. This is accomplished by varying at least
one of the following in accordance with the wavelength of the input light
beam (i.e., the specified wavelength): the amount of phase shift produced
by the variable optical phase shifter 18; the ratio of the first input
beam to the second input beam emerging from the optical splitter 14; and
the ratio at which the optical combiner 20 combines the output beams from
the optical intensity modulator 16 and from the variable optical phase
shifter 18.

[0087]Generally, the chirp characteristics of the optical intensity
modulator 16 vary with variations in the wavelength of the input light
beam as well as with variations in the ambient temperature. Therefore,
the optical modulation device 10 may be controlled based on the specified
wavelength so as to exhibit the desired performance characteristics
independently of wavelength. Especially, when the optical intensity
modulator 16 is a semiconductor Mach-Zehnder modulator or electro
absorption optical modulator, the characteristics of the optical
modulation device 10 vary depending on the difference between the
wavelength of the input light beam and the band gap wavelength of the
optical intensity modulator 16. Since the band gap wavelength of the
optical intensity modulator 16 varies with temperature, variations in the
wavelength of the input light beam can be accommodated in the same manner
as in the fifth embodiment.

[0088]It should be noted that the optical modulation device 10 of the
present embodiment may have the same configuration as in the first
embodiment. In such a case, the control circuit 48 adjusts the amount of
phase shift produced by the variable optical phase shifter 18 based on
the wavelength of the input light beam such that the chirp
characteristics of the output beam of the optical modulation device 10
are less wavelength dependent than those of the output beam of the
optical intensity modulator 16.

[0089]Further, the optical modulation device 10 of the present embodiment
may have the same configuration as in the second embodiment. In such a
case, the control circuit 48 adjusts the amount of phase shift produced
by the variable optical phase shifter 18 and/or the amount of attenuation
produced by the variable optical attenuator 28 based on the wavelength of
the input light beam such that the chirp characteristics of the output
beam of the optical modulation device 10 are less wavelength dependent
than those of the output beam of the optical intensity modulator 16.

Seventh Embodiment

[0090]FIG. 20 is a diagram showing an optical semiconductor device
according to a seventh embodiment of the present invention. The optical
semiconductor device, 42, is an optical module including an optical
modulation device 10 and lenses 52 and 54. The optical modulation device
10 of the present embodiment may be any one of the optical modulation
devices of the first to fourth embodiments. Light from an optical fiber
56 of an external optical system is introduced into the optical
modulation device 10 through the lens 52. The output beam of the optical
modulation device 10 is directed into an optical fiber 58 of an external
optical system through the lens 54.

Eighth Embodiment

[0091]FIG. 21 is a diagram showing an optical semiconductor device
according to an eighth embodiment of the present invention. It should be
noted that those components common to the seventh embodiment retain the
same reference numerals and will not be further described.

[0092]The optical semiconductor device of the present embodiment differs
from that of the seventh embodiment in that it additionally includes a
semiconductor laser 44 which is optically coupled through the lens 52 to
the optical modulation device 10 (hybrid integration). Thus, the present
embodiment allows for a reduction in the parts count of the module
including the optical source, as compared to the seventh embodiment,
resulting in reduced size of the optical transmitter.

Ninth Embodiment

[0093]FIG. 22 is a diagram showing an optical semiconductor device
according to a ninth embodiment of the present invention. In this optical
semiconductor device, the optical modulation device 10 and the
semiconductor laser 44 are monolithically integrated together. This
significantly improves the optical coupling efficiency from the
semiconductor laser 44 to the optical modulation device 10, thereby
enabling the intensity of the output beam of the optical semiconductor
device to be increased.

Tenth Embodiment

[0094]FIG. 23 is a diagram showing an optical semiconductor device
according to a tenth embodiment of the present invention. In this optical
semiconductor device, the optical modulation device 10 and a variable
wavelength multielectrode semiconductor laser 60 are monolithically
integrated together. This significantly improves the optical coupling
efficiency from the variable wavelength multielectrode semiconductor
laser 60 to the optical modulation device 10, thereby enabling the
intensity of the output beam of the optical semiconductor device to be
increased.

Eleventh Embodiment

[0095]FIG. 24 is a diagram showing an optical semiconductor device
according to an eleventh embodiment of the present invention. In this
optical semiconductor device, the optical modulation device 10, a
variable wavelength semiconductor laser array 62, and an optical combiner
64 are monolithically integrated together, the optical combiner 64 being
adapted to combine the beams of the lasers of the variable wavelength
semiconductor laser array 62. This significantly improves the optical
coupling efficiency from the optical combiner 64 to the optical
modulation device 10, thereby enabling the intensity of the output beam
of the optical semiconductor device to be increased.

Twelfth Embodiment

[0096]FIG. 25 is a diagram showing an optical semiconductor device
according to a twelfth embodiment of the present invention. This optical
semiconductor device includes a plurality of variable wavelength
multielectrode semiconductor lasers 60 (such as that of the tenth
embodiment), a plurality of optical modulation devices 10 (such as that
of the tenth embodiment), and an optical combiner 66. Each variable
wavelength multielectrode semiconductor laser 60 is connected in series
to a respective optical modulation device 10, thereby forming an arm, or
branch, as shown in FIG. 25. These arms are connected in parallel to one
another. The output beams of the optical modulation devices 10 are
combined by the optical combiner 66. Each variable wavelength
multielectrode semiconductor laser 60 covers a different wavelength band,
and the optical modulation device 10 connected to the laser operates at
that wavelength band. This arrangement allows the total wavelength range
of the optical semiconductor device to be arbitrarily increased by
increasing the number of arms (i.e., the numbers of variable wavelength
multielectrode semiconductor lasers 60 and optical modulation devices
10).

[0097]Obviously many modifications and variations of the present invention
are possible in the light of the above teachings. It is therefore to be
understood that within the scope of the appended claims the invention
maybe practiced otherwise than as specifically described.

[0098]The entire disclosure of a Japanese Patent Application No.
2008-168556, filed on Jun. 27, 2008 including specification, claims,
drawings and summary, on which the Convention priority of the present
application is based, are incorporated herein by reference in its
entirety.